1. Field of the Invention
The invention relates, in general, to pressure sensors, and more particularly, to a pressure sensors mounted in polymer housings.
2. Description of the Related Art
Semiconductor pressure sensors have been around for some time, and are generally packaged in either a one-piece or two-piece housing. In both of these types of packages, the actual transducer device generally is mounted on a base formed from a polymer material. U.S. Pat. No. 4,655,088 entitled, Unibody Pressure Transducer Package, discloses a one piece housing for mounting a semiconductor transducer. The housing is molded from a polymer material. Electrical leads pass through the one piece housing to provide electrical contact between the semiconductor transducer and outside circuitry. The transducer is mounted on a base portion of the polymer housing.
One problem with mounting a semiconductor transducer in a polymer housing arises due to a mismatch in thermal coefficients of expansion (TCE). The transducer ordinarily is secured to the polymer base using room temperature vulcanization (RTV). The RTV adhesive acts as a buffer to compensate for significant differences in the TCEs of the polymer housing and the semiconductor transducer. Differences in TCEs can be important. For example, it is conceivable that a packaged semiconductor transducer will be mounted on the inside of an automobile tire. Certain regions of the country such as, Michigan in the winter may be very cold, and other regions of the country, such as Florida in the summer may be very hot. The performance of the semiconductor transducer may be impacted by temperature changes which cause the semiconductor transducer and the polymer to expand or to contract at different rates consistent with their different TCEs.
Another problem which may be experienced by a semiconductor transducer is interference from radio frequency (RF) signals. Using the above example, a packaged semiconductor transducer may be secured within an automobile tire in order to monitor pressure within the tire. Pressure readings provided by the sensor may be read using radio frequency signals. For example, an external device may transmit RF signals to and receive RF signals from a circuit board on which the pressure sensor is mounted within the tire. These same RF signals, however, may impact the performance of the transducer. Hence, there may be a need to protect the transducer from the effects of RF signals.
A challenge in the manufacture of pressure sensors has been achieving wire bonds between metal leads which pass through a housing and a semiconductor transducer mounted inside the housing. U.S. Pat. No. 4,686,764 entitled. Membrane Protected Pressure Sensor, discloses a semiconductor sensor mounted in a housing and wire bonded to a metal lead. One technique for achieving the wire bonding is to heat a lead frame which bears the metal leads in order to heat the semiconductor transducer sufficiently to affect a bonding between the lead, the wire bond and the transducer. This technique typically requires a transfer of heat from the metal lead to the housing and then to the semiconductor transducer. Unfortunately, there can be difficulties with this mechanism for the transference of heat energy since the housing material may transmit heat less efficiently than the metal lead.
Thus, there has been a need for an improved semiconductor sensor in which there is a better match of TCEs of a semiconductor transducer and a base on which the transducer rests, which can better withstand the effects of RF interference, and which can more effectively use wire bonding techniques during production. The present invention needs these.
A pressure sensor is provided in which a metal base pad is molded into a polymer housing. A semiconductor transducer is placed on the metal base pad. A cap is attached to the housing to form a protective chamber in which the transducer is mounted on the base pad. A vent hole provides access to a measurand environment.
The metal base pad has a thermal co-efficient of expansion closely matched to that of the semiconductor transducer. Moreover. when the metal base pad is electrically grounded it provides a degree of protection from RF signals. Additionally, during wire bonding, the metal pad serves as an effective conductor of heat to the semiconductor transducer.